In fiber-to-the-curb (FTTC) or fiber-to-the-home (FTTH) optical communication systems, signals from a central office may be distributed to a plurality of optical network units (ONUs), located in the outside plant, which may also return signals back to the central office. For the purposes of this application, an ONU can represent a single user, such as a single household (FTTH), or an ONU can represent a group of users (FTTC), such as a number of households in a certain geographical area. Also, such a communication system can employ any type of distribution system, such as a switched telephone network and/or a broadcast cable distribution system. Further, the signals may be either analog or digital data signals.
To reduce costs of FTTC or FTTH optical systems, the cost of the highest price components in these systems are preferably shared among as many customers as possible. Since the transmitters and receivers in the central office are very expensive, the cost of these items are divided among many customers by splitting the downstream optical power emitted from each transmitter and routing the split portions to multiple ONUs. Also, the cost is reduced by combining the upstream optical power signals from multiple ONUs and routing the combined signals to the central office. Often, the signals to or from each ONU can be identified by assigning a specific time slot in the data stream unique to each ONU, thereby employing time-division multiplexing.
In order to share the transmitters and receivers among multiple ONUs, optical power splitters are required. A single feeder fiber is connected to each transmitter and the light in the feeder fiber is divided at the power splitter between the distribution fibers which distribute the light to the ONUs and also routes the light from the distribution fibers onto the single feeder fiber.
Another important method to reduce the cost of the FTTH/FTTC system would be to minimize the amount of fiber that needs to be installed in the outside plant. The amount of fiber is reduced by maximizing the length of the single feeder fiber, while minimizing the lengths of the multitude of distribution fibers. The sharing of transmitters and receivers and the lengths of the feeder fiber and distribution fibers require the use of a power splitter placed between the feeder fiber and the distribution fibers and placed in the outside plant near the ONUs.
Fiber optic communications infrastructures should ideally be able to evolve as the needs of the system change over time. For instance, a communication system may originally be designed for only a certain number of ONUs. Certain circumstances, such as new real estate development in the area, might cause a demand that far exceeds the present capability of the physical fiber plant and the present number and locations of the ONUs. To meet this new demand, the communication system must be expanded, such as by adding another transmitter, adding another power splitter, adding another optical line from the transmitter to the power splitter, and adding new lines from the power splitter to each additional ONU in the system. Because each of these additions to the communication system has an associated cost, a need exists for a communication system that can be easily expanded without incurring large expenses.
Another manner in which a communication system may need to evolve involves increasing bandwidth to the existing ONUs. While the number of ONUs may be within the capability of the system, the amount of bandwidth that an ONU requires may become more than the system is able to provide. At such a time, the system may increase bandwidth by transmitting at an additional wavelength. When the communication system has power splitters, the signals at the additional wavelength are divided and sent to all of the lines connected to the power splitter, even those which do not require the additional bandwidth. A need therefore exists for a communication system that can selectively provide dedicated services by expanding the bandwidth capabilities at only those ONUs which require the additional bandwidth.
Many optical communication systems employ methods for testing faults in the communication system. One commonly used testing method is an optical time domain reflectometry (OTDR) testing method. In general, every signal transmitted from the central office is echoed back to the central office due to signal reflections, sometimes referred to as Rayleigh scattering, at locations along the length of the fibers. The echoes at the central office are fairly uniform with the delay time associated with each echo corresponding to a certain length between the central office and the point of origin of the Rayleigh scattering. A fault in the system, such as a fiber break, can be detected by monitoring the delay times of the echoes. Reference may be made to U.S. Pat. No. 5,285,305 to Cohen et al. for more details on how an optical communication system can use OTDR to detect faults in the system.
The ability of OTDR to locate a defect precisely, however, is degraded when the communication system has power splitters. When a fault occurs past the power splitter on one of the lines connected to the power splitter, OTDR can detect the distance from the central office to the fault, but is unable to determine on which one of the two or more lines past the power splitter the fault occurred. To identify the location of the fault, service personnel must be sent to the various lines connected to the power splitter at locations which are at the detected distance from the defect to the central office. The time spent by the service personnel in locating the fault increases the time that the system in inoperable due to a particular fault and also adds to the overall cost of operating the communication system. A need therefore exists for a communication system which has power splitters, which can detect faults, and which can quickly and precisely locate the faults.